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IP Packet Sizing - MTU

How to Manually Set MTU in Windows by nocum
Posted: 3 Jun 02 (Edited 11 Jun 02)

Why:
The proliferation of high speed broadband (DSL, Cable, and Partial T1) connections has led to the replacement of dialup connections for Internet and VPN clients.  These connections normally require the use of a network card on a Windows OS system employing the TCP/IP protocol.  The default MTU in Windows is 1500 when speeds are over 128kbps (kilobits) where the MTU for dialup is 576 (re: below 128kbps).  It is remotely possible your MTU will be forced to 576 by a router somewhere between your connection and your desired destination.

The whole concept behind broadband is speed, therefore anything that reduces speed is bad, well not acceptable.  When your PC routes a packet larger than the MTU of your broadband provider and any router between, it is fragmented into two, and possibly more packets.  This reduces speed and can play havoc with VPN connections û no connects, drops, etc.  Determining and adjusting the MTU on your PC can provide you with the speed that you are paying for while eliminating a common problem with VPN connectivity and stability.

There are ôtweakingö software packages out there that profess to do it all for you.  I cannot attest to their effectiveness and accuracy.

As of June 11, 2002 I have witnessed extreme variations in the MTU with my DSL connection.  It appears that it is due to the routing of my packets.  I have elected to adopt 576 as my MTU as a "safe harbor" setting as a result.  This FAQ will updated to reflect the resulting performance.

OS Covered:
Windows 95/98
Windows NT
Windows 2000

Prerequisites:
You must have determined the optimum MTU for your connection (connections if you are using a VPN) before applying the procedures in the document.  See the document ôHow do I determine the maximum packet size for DSL and Cable modem Internet Connection (critical for VPN host & clients)?ö for instructions on how to do this.

Details:
You will have to set it manually by adding the proper registry entry and with the tested MaxMTU value into the client system only.

Preparation:
-Windows 95/98--
Close all applications.  Click on Start > Run > type in
"winipcfg" > click the Ok button.  In the window that appears is the current IP address assigned to the selected device.  If it says PPP Adapter, you must click on the down arrow to the right and select the Ethernet Adapter.  Write down the IP address that appears.

--Windows NT/2000--
Close all programs.  Click on Start > Run > type in "cmd" >
click the Ok button.  In the command prompt window that appears type in "ipconfig /all" and press the ENTER key.  Scroll up and down to find the IP address assigned to your network card(s). Write it/them down.

Edit the Registry:

-WARNING-WARNING-WARNING-WARNING-WARNING-WARNING-WARNING-
Making incorrect changes to the System Registry may cause your system to not operate properly. I strongly recommend someone familiar with the Microsoft System Registry make these changes. I highly recommend backing up your system registry before making manual changes to it for this or any change.  If you don't know how to start editing the registry, then you shouldn't be attempting this (or have your resume ready to mail out).

---Windows 95/98---
Click on Edit > Find and enter "MaxMTU" (make sure you examine all registry entries).  In most instances it will not be found, so navigate to:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Class\NetTrans\000x(identifiable by the IP address written down earlier)

Right click on the right side of the screen in the blank area and click on Add > STRING  Value> enter MaxMTU > Ok.  Right click on the MaxMTU and click on Modify.  Enter the value determined using the ping command earlier > Ok.   You should see something like MaxMTU="####" (STRING Value (Reg_SZ)).  Click on  Registry then Exit to close and save the registry.  Reboot this system properly.
 
---Windows NT---
Click on Edit > Find and enter "MTU" (make sure you examine all registry entries).  In most instances it will not be found, so navigate to:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\<Ethernet-adapter name>\Parameters\Tcpip\
(identifiable by the IP address written down earlier)

Once you have found the proper registry entry, right click on the right-hand side of the screen in the blank space > New > DWORD Value > type "MTU" > Ok.  Right click on "MTU" >  Modify > Decimal > the value determined using the ping command earlier > Ok.  You should see something like MTU=#### (Reg_DWORD).  Click on Registry then Exit to close and save the registry.  Reboot this system properly.
 
---Windows 2000---
Click on Edit > Find and enter "MTU" (make sure you examine all registry entries).  In most instances it will not be found, so navigate to:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\
(identifiable by the IP address written down earlier)

Once you have found the proper registry entry, right click on the right-hand side of the screen in the blank space > New > DWORD Value > type "MTU" > Ok.  Right click on "MTU" > Modify > Decimal > the value determined using the ping command earlier > Ok.  You should see something like MTU=#### (Reg_DWORD).  Click on  Registry then Exit to close and save the registry.  Reboot this system properly.

-Unix Family-
Perhaps a guru partial to Unix variants will write us all a how-to document similar to this one.

-AuthorÆs Note-
This document represents a best effort on the part of the author to provide assistance in a ôhow-toö desert.  It is submitted on the basis of ôIt worked for me.ö  It may not work for the reader.  Use these guidelines at your own risk.  If you are not technically adept, then you are advised to seek professional technical assistance.  If this FAQ helps one professional keep a job or client then it has been worth the time.  I was not so fortunate.

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